PURPOSE: To obtain a sound deadening effect, which is acquired from a miniaturized silencer, the cost of which is reduced and which is stable, without taking space by closely attaching and arranging an electroacoustic transducer generating a sound for muffling to a noise generator, muffling the periphery of the noise generator by an acoustic-wave interference and uniformly damping noises in an engine room.

CONSTITUTION: When a compressor 7 is operated, a pickup 9 detects the vibrations of the compressor 7, and an adaptation type muffling control section 15 is supplied with the detecting signal. The adaptation type muffling control section 15 prepares a control sound signal having antiphase and the same amplitude, and supplies a speaker 11 with the signal. When the control sound is generated from the speaker 11 closely attached and disposed to the compressor 7, an acoustic-wave interference is generated in an engine room 5, and the emission of a sound from the surface of the compressor 7 is reduced extremely, thus obtaining a sound deadening effect.